Guest Author - Tabitha Brewer

FROST

On those chilly mornings when you go outside and start your car in order to defrost your windows, do you ever wonder what causes frost? I do, so I did a little research and this is what I found:

In order for frost to occur, there are two conditions that must be met. There must be sufficient water vapor in the air and the dew point temperature must fall below freezing (32ºF). When these two conditions are met, frost occurs through a process called deposition. Deposition is the conversion of water vapor into ice, skipping the liquid phase all together. When the there is sufficient water vapor in the air, but the dew point temperature does not fall below freezing (32ºF), then dew is formed.

There are two main types of frost: Hoar frost and Rime frost. What we experience in this part of the country (eastern Kentucky) is Hoar frost. Hoar frost forms through the slow deposition of water vapor directly on a surface as ice. It forms best when winds are light, which usually occurs on a clear, cold night. Hoar frost accumulates slowly and forms delicate interlocking crystals that grow outward from the surface with a feather, fern or flower pattern. It grows in steps or layers. Hoar frost appears white because small air bubbles are trapped in the ice and reduces its transparency. It glitters in the sunlight, particularly early in the morning as the sun rises.

Rime frost, on the other hand, is found in the more mountainous regions such as Colorado and Montana. Rime frost occurs rapidly, changing from the liquid stage to the ice stage quickly. It forms best when winds are moderate and there is high atmospheric water content. It gives a dull, matte finish to the surface it forms on. Rime frost is most common during cold fogs, when water droplets come in contact with subfreezing surfaces or when moisture-laden clouds are forced over cold mountain slopes. Rime frost grows toward the wind direction. Accumulations of Rime frost can reach a thickness of three feet or more.

When frost occurs depends largely on latitude. Areas that are farther to the north reach 32ºF sooner than areas south. Valley locations can receive the first frost before the ridges. For example, the Jackson, Kentucky National Weather Service Office with an elevation of 1381 feet can expect to see 32ºF around October 26th while West Liberty, KY with an elevation of 830 ft. can expect to see 32ºF around October 6th. The National Weather Service issues two products, a Frost Advisory and a Freeze Warning, when dealing with the cooler temperatures. A Frost Advisory is issued when widespread frost is expected and is issued until a killing freeze occurs. A Freeze Warning is issued when temperatures are expected to be below 32ºF over a widespread area. Once a killing freeze occurs, the Freeze Warning is no longer issued.